Capacitance sensors are used to implement a variety of useful functions including touch sensors (e.g., touch pad, touch dial, touch wheel, etc.), determining the presence of an object, accelerometers, and other functions. In general, capacitive sensors are intended to replace mechanical buttons, knobs, and other similar mechanical user interface controls. A capacitive sensor permits eliminating complicated mechanical switches and buttons, providing reliable operation under harsh conditions. Capacitive sensors are widely used in the modern consumer applications, providing new user interface options in existing products (cell phones, digital music players, personal digital assistances, etc.).
One class of capacitive sensor uses a charge transfer technique. Referring to FIG. 1, the charge transfer technique charges a sensing capacitor Cx in one phase (switch SW1 closed, switch SW2 open) and discharges the sensing capacitor Cx into a summing capacitor Csum in a second phase (SW1 open, SW2 closed). Switches SW1 and SW2 are operated in a manner to repeatedly transfer charge from Cx to Csum.
Capacitance sensor 100 is operated to measure the capacitance of Cx in the following manner. In an initial stage, Csum is reset by discharging charge on Csum by temporarily closing switch SW3. Then, switches SW1 and SW2 commence operating in two phases that charge Cx and transfer the charge from Cx into Csum. The voltage potential on Csum rises with each charge transfer phase, as illustrated in FIG. 1B. The capacitance of Cx is determined by measuring the number of cycles (or time) required to raise Csum to a predetermined voltage potential. Alternatively, the capacitance of Cx can be determined by measuring the voltage on Csum after executing a predetermined number of charge transfer cycles.
Relative to other capacitive sensing techniques, the charge transfer method has relatively low sensitivity to RF fields and RF noise. This relative noise immunity stems from the fact that the sensing capacitor is typically charged by a low-impedance voltage source and the charge is transferred to a low-impedance accumulator (i.e., the summing capacitor Csum). However, the charge transfer technique is still susceptible to RF noise due to potential RF signal rectification by electrostatic discharge (“ESD”) protection circuits inside an integrated circuit (“IC”) implementation. Furthermore, capacitance sensor 100 is sensitive to DC currents (e.g. leakage) on sensing capacitor Cx. These DC current may arise from printed circuit board (“PCB”) or sensor assembly leakage problems, high power UHF signals (e.g., cell phones, microwave ovens, etc.), or rectification by on-chip electrostatic discharge (“ESD”) protection diodes. Additionally, the charge transfer mechanism is susceptible to noise having a frequency matching (or harmonics thereof) the switching frequency of switches SW1 and SW2 due to an aliasing phenomenon.